GB312468A - Improvements in or connected with sound reproducing and recording apparatus - Google Patents

Abstract

312,468. Johnson, C. H. May 4, 1928. Magnetic instruments.-In an electromagnetic instru- , ment of the vibrating armature type, more particularly a gramophone pick-up, the main restoring force on the armature is that due to an auxiliary magnet pole, the axis of vibration of the armature being further from the auxiliary pole than from the main poles carrying the winding, and preferably on the side of the latter remote from the former. In addition the axis of vibration preferably passes sub. stantially through the centre of inertia of the vibratory system. The E-shaped permanent magnet 1, Fig. 1, has its central auxiliary pole 10 of one polarity and its outer poles, on which are mounted pole-pieces 9 and coils 17, of opposite polarity. The armature 11 is carried on a spindle 12 which fits into holes lined with rubber bushes 13 and it vibrates to and from pole-pieces 9, while an extension vibrates transversely across auxiliary pole 10. The tip of the extension and the auxiliary pole face are preferably of the same width and the gap between them is preferably small and carries the whole of the magnetic flux. In a modification, Fig. 4, the main pole-pieces are laminated and the armature is mounted in soft rubber pads 19, between the plates 20, the rubber being allowed to project slightly into the space 21 around the spindle, forming a bush for the holes in plates 20. Another armature mounting, Fig. 6, comprises rubber bushes 13, surrounding the armature spindle 12, and resting in containing caps 22, which are secured to the back plate of the pick-up and are adjustable axially. The permanent magnet may be replaced by an electromagnet, in which case the restoring force due to the auxiliary pole may be varied, or this may be accomplished in the case of the permanent magnet, according to the Provisional Speci. fication, by short-circuiting or diverting by known means part of the magnetic flux from the auxiliary gap. In either case the flux variation serves also as a volume control.